Formation of polarized epithelium is fundamental to embryonic development. During mouse peri- implantation development, the undifferentiated inner cell mass of the blastocyst is converted from a nonpolar cell aggregate to a highly organized epithelial cyst. This morphogenetic transformation involves the polarization and basement membrane (BM)-dependent survival of epiblast epithelium. However, the molecular mechanisms underlying these processes are largely unknown. The mouse peri-implantation development can be recapitulated by in vitro cultured embryoid bodies (EBs) differentiated from embryonic stem cells. Using genetically modified EBs, we show for the first time that BM formation directs the assembly of an adhesion complex, which serves as a signaling platform to regulate epiblast polarization and survival. Our preliminary data supporting this idea, demonstrating that (1) assembly of the BM-associated adhesion complexes correlates with the activation of Rho GTPases Cdc42 and Rac1;(2) Cdc42 controls microtubule organization and the trafficking of apical polarity proteins, processes that are required for cell elongation and polarization;and (3) Rac1 activation plays an essential role in BM-dependent epiblast survival. These data strongly support our hypothesis that BM formation induces epiblast polarization and promotes epiblast survival via activation of Cdc42 and Rac1. To test this hypothesis, we propose to determine the mechanism through which the assembly of BM-associated adhesion complexes induces Cdc42 and Rac1 activation (Specific Aim 1 and 3). Next, we will determine the Cdc42-effector interactions that regulate epiblast elongation and apical polarization, focusing on the role of IQGAP1 in microtubule capture and the exocyst in apical polarity protein trafficking (Specific Aim 2). Finally, we will determine the Rac1- effector interactions that regulate epiblast survival (Specific Aim 4). The long-term goal of this study is to identify the extracellular cues and the transmembrane cascades that regulate embryonic epithelial morphogenesis, which are crucial to our understanding of epithelial biology, embryogenesis, and tissue regeneration. Public Health Relevance: Elucidating the molecular mechanisms of embryonic epithelial tissue formation that integrate cellular polarity, differentiation and survival with tissue architecture is not only critical to our understanding of embryonic development and cancer biology but also has implications in tissue regeneration and tissue engineering.